In recent years, developments of a configuration using, as a light source of a backlight used in a liquid crystal display apparatus, a light emitting diode (hereinafter referred to as “LED”) instead of a conventional cold cathode fluorescent tube (hereinafter referred to as “CCFL”) have been advanced. Especially, as compared with a conventional CCFL tube backlight, an LED backlight which uses LEDs of three primary colors (red, green and blue) is characterized in that its color reproduction region (NTSC ratio, CIE1931) is large. This is because a half band width of an emission spectrum of the LED is narrow, and RGB colors which are respectively close to their pure colors can be obtained. Moreover, since the brightness of the LEDs of each color can be adjusted by the amount of current, balance among the colors can be changed. Further, unlike the CCFL, the LED does not contain mercury, so that it excels in environment-friendliness.
In addition, LED lighting apparatuses including LEDs are increasingly used as light sources for general lighting other than for liquid crystal display apparatuses. Especially, since an apparatus including LEDs of three primary colors (red, green and blue) can control emission intensities of three primary colors, it is possible to stabilize or change a color temperature of the light source.
However, regarding the LED lighting apparatus including the red LEDs, the green LEDs and the blue LEDs, when the brightness of the three colors changes or the balance among the three colors changes, the brightness and/or color of an object to be illuminated changes. When using the LED lighting apparatus as the backlight of the liquid crystal display apparatus, the number of LEDs used is large, so that slight changes in the brightness or color of each LED are perceived as uneven luminance or uneven color of the entire liquid crystal image.
Currently, to obtain high luminance, it is desirable to use an AlGaInP based semiconducting material as the red LED, and an AlGaInN based semiconducting material as the green LED and the blue LED. Since the half band width of the emission spectrum of the red LED using the AlGaInP based semiconducting material is narrow (about 20 nm), this red LED excels in monochromaticity. By using this red LED, it is possible to increase the color reproduction region of the liquid crystal display apparatus. However, a lowering rate of the luminance of the AlGaInP based red LED at the time of temperature rise is higher than that of the AlGaInN based LED. Moreover, a change in a peak wavelength of the emission spectrum of the AlGaInP based red LED is larger than that of the AlGaInN based LED. Further, as the temperature increases, the emission spectrum of the red LED shifts toward a long wavelength side. In light of visibility characteristics of humans, this change is a wavelength shift that is deterioration for the visibility. Therefore, even if the emission intensity of the AlGaInP based red LED is the same as that of the AlGaInN based LED, the AlGaInP based red LED looks dark to the human eye. Further, since the emission intensity itself of the AlGaInP based red LED decreases as the temperature rises, two disadvantageous factors occur, and this leads to decrease in the emission intensity of a red component perceived by the human eye as the backlight. In light of the coloring of the backlight, this decrease relatively increases percentages of blue and green components, so that the color reproducibility is deteriorated. To compensate for the decrease in the red component, the current flowing to the LED has been increased. However, this causes problems, such as increase in power consumption, increase in the amount of heat generated, and decrease in reliability.
To deal with these problems, FIG. 1 of Japanese Unexamined Patent Publication No. 222904/2001 (Tokukai 2001-222904, published on Aug. 17, 2001) shows a light source which uses light emission from a phosphor which converts exciting light of the blue LED to generate red light emission. This significantly mends the problem of the change in a color tone due to the temperature change.
FIG. 1 of Japanese Unexamined Patent Publication No. 118292/2002 (Tokukai 2002-118292, published on Apr. 19, 2002) shows a light source which emits three primary colors using blue LEDs as excitation light sources, and green and red phosphors.
FIG. 3 of Published Japanese Translation of PCT International Publication for Patent Application No. 515956/2003 (Tokuhyo 2003-515956, published on May 7, 2003) shows an LED lighting system which uses a light source including, for example, red LEDs, green LEDs and phosphors which are excited by blue LEDs to emit red light.
When using, as a red phosphor, 0.5MgF2.3.5MgO.GeO2:Mn shown in FIG. 2 of Japanese Unexamined Patent Publication No. 171000/2002 (Tokukai 2002-171000, published on Jun. 14, 2002), the half band width of the red phosphor is narrower than that of the red LED, so that it may be possible to obtain the excellent color reproduction region. Meanwhile, the pamphlet of International Publication No. 2005/052087 (published on Jun. 9, 2005) reports that a wavelength conversion efficiency of a nitride based phosphor is excellent. The half band width of the emission spectrum of this nitride based phosphor is wider (about 60 nm) than that of the red LED. Therefore, this phosphor is excellent as general lighting, but has been thought to be not suitable for a liquid crystal backlight whose color reproduction region is large.
The light emitting apparatus which uses only LED chips excels in the color reproducibility, but the problem is that the luminance and chromaticity of the LED changes largely by the temperature change. Meanwhile, the light emitting apparatus in which LED chips of one specific color are replaced with phosphors excited by an LED light source(s) can improve the stability of the luminance and chromaticity with respect to the temperature change. However, when the half band width of the emission spectrum the phosphor is wide, the problem is that the color reproducibility of the display apparatus deteriorates.